Composition board with improved polymeric skin

ABSTRACT

A composition board which comprises a thin nonfrangible sheet material suitable for use as a surface covering, which material comprises: a fibrous sheet material impregnated with a cured thermosetting resin and characterized by one surface which is a nonporous, adhesive-free, fully cured surface, and another surface which is characterized by an adhesive-rich nonsanded surface, the adhesive being compatible with the cured resin, the adhesive-rich surface adapted to be bonded to a substrate, and the resin-rich surface used as a face surface for the substrate.

This is a division of application Ser. No. 693,198, filed June 7, 1976(now U.S. Pat. No. 4,083,743, issued Apr. 11, 1978).

BACKGROUND OF THE INVENTION

Composition boards like particle boards or other substrates with a thin,hard, formic-like top surface covering are well known. Typically, thesurface comprises a cured thermoset resin in a translucent, opaque orprinted sheet which is secured to one surface of the board to provide awear- and stain-resistant surface, such as, for example, in kitchencounter tops, wall coverings, furniture and the like.

A prior art standard method (the high-pressure method) of preparing suchcomposition boards has been to prepare a thick resin-cured laminateoverlay sheet typically with four or more underlay sheets ofresin-impregnated kraft paper. The overlay sheet is prepared under highpressure and resin-curing temperatures, and the back surface of thelaminate sanded or roughened. The laminate, for example, with aconventional thickness of about 0.35 inches, is then glued by means oflow or contact-pressure adhesives to the composition board surface. Thismethod requires multiple underlay sheets to provide a cured laminateoverlay of sufficient strength to be handled and sanded.

Another technique for producing such composition boards (thelow-pressure method) is also used, but likewise has severaldisadvantages.

In one method, a board core; that is, usually a thick layer of woodchips bonded together by a resin with a thickness ranging from about0.25 cm to 5.0 cm or more, is used as a base, and a thin, transparent,opaqued or printed sheet, which is impregnated with a curable thermosetresin, such as a melamine-formaldehyde resin, is bonded to the topsurface of the board by subjecting it to heat and pressure sufficient toeffect simultaneously the curring or fusing of the melamine-formaldehydereins in the sheet.

In such operation, care must be taken not to employ too much pressure,since pressure, for example, in excess of 10 or 20 kilos per squarecentimeter or above, often alters or weakens the mechanical andstructural properties of the underlying board. In any event, the use ofsuch pressure always reduces the thickness of the board in some degrees.Care must be taken also in the selection of the board, since a boardusually has a relatively rough surface created by larger particles orchips of wood and, for example, of more than about 0.3 mm± variation inthickness, would be unacceptable, since the underlying surface defectsof the board may be telegraphed through the thin sheet to causeundesirable ripples and surface defects on the top cured surface.

Attempts have been made to overcome this defect by employing wood chipsof particularly graded particle size on the surface of the board, butthis is expensive and involves another process step. In addition,efforts have been made to smooth; e.g., to presand or presurface, theboard surface to make it more acceptable and to raise it into ahighly-quality surface board, which is also expensive.

A method of overcoming the problems of telegraphing has been to select avery short heat cycle and to restrict the amount of pressure involved;for example, a heat cycle of only 30 to 90 seconds, and a pressuretypically below 20 kilos per square centimeter, such as 5 kcm or lower.By this operation, telegraphing and deterioration of the particle boardby heat is minimized and reduction in thickness of the board isminimized also. The top surface of the board prepared in this manner,however, does not present a fully cured resin surface on the thin sheet,but rather a less than fully cured thin top sheet which is more porousin nature than a fully cured sheet, which is subject to high heat andhigh pressure and longer curing times. The top surface, by being moreporous and not fully cured, does not have the high-heat andtemperature-resistant properties which would be desirable, and such topsurfaces exhibit a reduced resistance to chemicals and stains and alow-resistant surface to abrasion, wear and scratches which isunsatisfactory for many uses.

A further technique employed to overcome the problems associated withtelegraphing by the rough surface of a board has been to employ arelatively thick kraft or alpha paper which is impregnated with athermoset curable resin as an underlay between the thinresin-impregnated top sheet and the rough surface of the board. By thisprocess, lower-quality or rough-type particle boards can be employed andtelegraphing prevented. One difficulty associated with the technique isthat it involves extra expense and cost through the use of the kraft oralpha paper underlay, and further there is often a visible"show-through" of the phenolic resin impregnated in the kraft paper. Inaddition, although this technique prevents telegraphing, it still doesnot permit the use of very high pressures nor the preparation of a fullycured and high stain-, scratch-, abrasion- and wear-resistant topsurface in the low-pressure process.

An economic disadvantage associated with the low-pressure process isthat any surface defects discovered in the thin top surface result inthe discarding of the entire board or at least a reduction in size ortrimming of the entire board. Therefore, it is most desirable to preparea composition board having a thin, fully cured, polymeric, thermosetskin without the disadvantages of the foregoing low- or high-pressuretechniques, and which board with the thin skin will exhibit high-heattemperature, high stain, scratch and abrasion resistance.

SUMMARY OF THE INVENTION

My invention relates to an improved, flexible, fully cured,resin-impregnated sheet material suitable for use as a surface on acomposition board or other substrate, to an improved laminate, such as acomposition board, prepared with my cured sheet material securedthereto, and to the method of preparing the cured resin sheet materialand the improved laminate.

I have discovered that a curable, thermosetting resin, such as acondensable amino resin, like a melamine-formaldehyde resin, impregnatedin a transparent, opaque or printed fibrous sheet material, can, in aseparate operation, be subject to high heat and pressure conditions toproduce a fully cured, nonporous, flexible sheet material. Thereafter,the fully cured resin sheet material can be bonded at room or lowtemperatures and at low or contacting pressures to a substrate surface;e.g., a composition board surface. I have found that subjecting a thin,melamine-resin-impregnated alpha sheet material to high pressures and acuring temperature in a separate operation provides for a very thin,fully cured sheet material. However, the sheet material was incapable ofbeing used, since it was quite frangible in nature, sometimes almostlike glass, so that it could not be sanded and handled in the ordinarycourse of manufacture or be secured by bonding to the surface of aboard. Attempts to incorporate plasticizers or fillers into the resinused in impregnating the sheet material were not satisfactory, sincesuch addition of materials reduced the quality of the cure and thechemical and physical properties of the resulting thin sheet material.

However, I have now found that, if a layer of an adhesive material, suchas polyvinyl acetate, compatible with thermosetting resins likemelamine-formaldehyde resins, is applied, such as by coating ortransferring, to one surface of a thin resin-impregnated alpha-typesheet material, and the sheet material is then subject to high pressuresand curing temperatures, the resulting sheet material is a flexiblesheet material and may easily be handled, and, thereafter, bondedwithout sanding in a separate bonding operation to the surface of aparticle board. The sheet material produced is thus a very thin fibroussheet material which contains a fully cured, thermosetting, resin-richsurface on the face surface and an adhesive, like polyvinyl-acetate ofadhesive rich, surface on the back or bonding surface. Apparently,although not wishing to be bound by any particular theory, thecompatible adhesive flows and is forced into the resin fibrous sheetduring the high-pressure curing cycle and provides for a flexiblecarrier sheet, while providing an adhesive-rich surface for later aid inbonding of the sheet directly to the surface of a composition boardwithout sanding of the back surface.

By my technique, the thin resin-impregnated sheet can be subject to anydesirable high pressures and temperatures in order to insure what theimpregnated resin in the sheet material is fully and adequaterly cured,and not only fused as in short heat-cycle operations. My sheet avoidsthe problems associated with compacting the board at high pressureswhich weaken or seriously damage the board if carried out while thesheet is on the board. Typical pressures which may be employed in mymethod would range from over 20 kilos per centimeter square (k/cm²),such as from 30 to 100 k/cm². In addition, due to the thinness of thesheet material and since the entire board does not have to be heated orbe subjected to high pressure, many more of my thin sheet materials maybe placed in the opening of the high-pressure press and heated for alonger period of time, such as from 3 (for catalyzed resins) up to 60minutes (for uncatalyzed resin) if desired; e.g., 10 to 20 minutes, forexample, at 104 to 121° C., depending on the number of sheets in oneopening of the press, the heating-up and cooling-down time of the moldsand press plates, in order to insure that the resin is fully cured. Thecured sheets provide a nonporous surface which has higherheat-temperature, stain, chemical, abrasion, scratch and wear resistanceand better impact strength than produced by shorter heat cycles at lowerpressures, as in the low-pressure process.

In addition, another important advantage of my invention is that theresulting flexible sheet can be examined and defects removed prior tobonding to the board without discarding the underlying board, therebyreducing cost and waste. Furthermore, as will be evident, the productionefficiency and rate can be considerably increased in a high-pressurepress, since the thin impregnated resin sheet would only be subject tohigh temperatures and curing temperatures without the presence of theentire board, thereby permitting a higher production rate.

My technique and invention also permit the easy handling of the curedresin sheet prior to bonding to the board surface. The use of mymaterial does not necessitate the use of a kraft or other underlaysheet, as in the high-pressure process, or with rough surface; although,if desired, such underlays may be employed. My fully cured skin sheetmaterial permits the use of lower-quality particle boards or othermaterials with rougher surfaces than previously used, since the adhesiveglue material employed can fill in the variations in the rough surface,and telegraphing of the surface variations is minimized.

Thus, particle boards having surface variations of up to ±0.3 mm can beused in the preparation of high-pressure laminates with compositionboards, such as particle boards. Fine-particle or carefully graded woodchips as a layer on the surface of the particle board to obtain a smoothsurface need not be used, since my resin-cured skin layer may be simplyglued on at very low or with only contacting pressures and temperatures,and, the adhesive glue line compensates for the imperfections, if any,in the surface of the board. Also, my thin sheet material could not besanded without breaking; however, it does not require sanding orroughening of the adhesive-rich back surface, as the thick laminateoverlay sheets of the high-pressure process.

An important addition, of course, is that, by the use of only contact orlow pressures in bonding on my cured resin-impregnated sheet material tothe board, the thickness of the board is not reduced, as in pastlow-pressure methods, by higher pressures, but the thin sheet is, infact, an added thickness layer to the board. The disadvantages of theprior method, when a particle board is subject to high pressures andtemperatures which alter the mechanical and structural features of theboard, are avoided, since the bonding pressures employed do not causeany change in the underlying board.

The sheet material employed is typically a very thin; for example, about0.1 to 0.3 mm, fibrous; e.g., cellulosic, sheet material having a basisweight of from 15 to 180 grams per square meter (gsm); e.g., 80 to 120gsm; although higher weights can be used if desirable, but it is moreexpensive. The sheet is impregnated with a curable, cross-linkable orthermosetting resin material, and often is an alpha cellulosic papersheet. A wide variety of curable resins may be used to impregnate thesheet, such as a melamine resin, such as a melamine-formaldehyde orurea-formaldehyde resin, as well as other water- or alcohol-soluble,condensable amino resins and phenol-formaldehyde resins andcocondensable mixtures thereof, alone or with other materials andadditives used in such resin formulations. Other heat-curable,condensable, thermosetting resins may also be used. The resin content ofthe dried sheet ranges from about 20 to 70% by weight, depending on thebasis weight of the sheet used. The impregnated sheet may be relativelytransparent or translucent, or it may be opaque, colored or have aprinted design therein which is decorative on the surface. The sheetmaterial is very thin and often only one sheet is employed; althoughseveral sheets of varying designs or combinations or a transparent topsheet and an underlying printed sheet may be used, and the sheets may beused alone or in combination with underlying sheets of other fibers,such as a curable resin-impregnated kraft or alpha or other paper orunderlay material.

The adhesive or glue employed may comprise a wide variety of natural orsynthetic adhesive materials, such as a vinyl ester of a short-chainacid C₂ -C₆ like a vinyl-acetate resin, for example, in an alcohol orwater emulsion or in a water or organic solvent solution. Any compatibleadhesive or polymeric-type resin, having adhesive properties which arecompatible with the resin employed in the thin sheet material, may beemployed. Polyvinyl-acetate resin (PVA) is the preferred material.

The adhesive layer is either coated onto or transferred to the onesurface of the thin sheet material by a transfer mechanism in a desiredthickness which permits the lower surface to become adhesive-rich, butinsufficient in amount to permit the adhesive to flow and passcompletely through the thin sheet material and appear on the topsurface. The adhesive typically impregnates less than about a majorityof the depth of the thin sheet, such as up to about 25% of the depth ofthe thin sheet.

My cured sheet material may be secured by the selected adhesive, or anycombination of adhesives, to any substrate surface, particularly acellulosic-based hard surface or supporting surface, such as hard board,plywood, natural wood, wallboard, composition board comprising woodfibers and hardened resin, such as a particle board, and similar sheetmaterials which are employed in use for furniture, counter tops, wallsurfaces and the like. The cured sheet material may be adhesively bondedto one or both surfaces of the substrate.

For the purpose of illustration only, my invention will be described inits preferred embodiment, wherein a cured sheet material is adhesivelybonded by a particular adhesive to the top surface of a particle board.However, it is recognized and is within the spirit and scope of myinvention that changes and modifications may be made in the describedembodiment, by those persons skilled in the art, which are within thescope and understanding of my invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of my process of manufacturing mycurable sheet materials.

FIG. 2 is a schematic illustration of an enlarged exploded view of theprocess for curing my sheet material of FIG. 1 in a press.

FIG. 3 is a schematic cross-sectional illustration of the process ofsecuring my cured sheet material to the surface of a particle board.

FIG. 4 is a representative fragmentary enlarged cross-sectional view ofthe finished particle board laminate product produced by the process ofFIG. 3.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is an illustration of the process of preparing a curable sheetmaterial of my invention which is characterized by a one surface being acured resin-rich surface, and the other surface being an adhesive-coatedsurface, wherein a decorative sheet material to be impregnated 10, suchas an alpha cellulose sheet, basis weight 35 to 200 g/cm², is prewet onone surface with a resin solution 12 in a container 14 which comprisesan aqueous solution of a heat-curable melamine-formaldehyde (MF) resinsolution. The resin solution 12 is applied by a resin applicator whichcomprises a pair of rollers 18 and 20 and a doctor knife 16 or by othercoating applicator means. The resin prewet sheet 22 is then impregnatedwith the same or a different curable thermosetting resin, such as amelamine-formaldehyde resin solution 26, by a roller 24, which immersesthe sheet in the container of the resin solution. The "pick-up" of theresin on the sheet is controlled by squeeze rollers 27 shown or by awire bar and/or by an air knife. The resin-impregnated sheet 28 is thencoated on one surface with an MF-compatible adhesive material, such as apolyvinylacetate (PVA) resin emulsion 36, through rollers 30 and 32 witha doctor knife 34, wherein a coating of the adhesive is applied to oneside of the sheet material 28.

The PVA-coated curable resin-impregnated sheet 40 is then dried in ahot-air oven 42 or by other means, such as radiant heaters, or acombination thereof, to drive off the water and/or any volatile organicvapors where aqueous or organic solvent solutions or emulsions areemployed and for partial condensation of the resin, with the oventemperature typically from about 110 to 200° C.; e.g., 140 to 160° C. Acurable, flexible, nonfrangible sheet material 44 is recovered, whichmaterial is characterized by a curable MF resin-rich surface, with thecurable resin impregnated throughout the sheet, and the opposite surfacecoated with a dried PVA coating of the desired thickness; e.g., 1 to 20mils.

FIG. 2 shows the curing of the sheet material 44 in ahigh-temperature/high-pressure press to provide a fully cured, thin,flexible, nonfrangible sheet material. A pair of steam or electricalheat-mold plattens 50 and 52 are employed in an arrangement with padswhich serve as insulating- and heat-distributing pads for better or evenheat distribution, in combination with release sheets as requiredbetween the sheet material. The press plates, typically of stainlesssteel with a polished finish surface, enclose a plurality of curablesheet materials 44 placed in the opening of the press.

In operation, the sheet materials 44 in the press are subject topressures of over 20 k/cm² ; e.g., such as 25 to 40 k/cm² ; e.g., 35k/cm², at temperatures of about 90° to 180° C, such as 105° C. to 150°C., for a time, such as 10 to 20 minutes, to provide a fully cured,nonfrangible, flexible sheet material 46. The operation forces thecoated adhesive material 36 into the one surface of theresin-impregnated sheet 44, so that the resulting material 46 ischaracterized by a dried PVA-rich surface and a cured MF-resin-richsurface. The adhesive is forced into the one surface to a desired depth,but insufficient to force the PVA to the cured resin-rich surface, suchas about from 10 to 40% of the sheet depth. The cured flexible sheetmaterial is now ready for use as a surface in a laminate product. Theresulting sheet material 46 may be handled in the normal course ofmanufacture without excessive cracking or crazing due to itsnonfrangible nature, an importantly without the necessity of sanding forbonding. The sheet material 46 may be inspected for defects at thisstage, and then stored typically in sheet form for use later inpreparing laminates.

The overlay press cycle to be employed depends on a number of factors,including, for example, the temperature of the heating medium, thetemperature and velocity of the cooling water, and also the number ofsheets placed in an opening of the press. Unlike the prior arthigh-pressure process, a large number of my thin sheets can be placed inthe press opening; e.g., 30 to 60 sheets per opening, and thus in amultiple-opening press, overall production of the sheets is enhanced,even though the press cycle is longer than in the prior art low-pressureprocess.

The mechanical cycle is the time period between the closing and openingof the press, while the heat cycle is the time period during which thesheets are subject to the resin-curing temperature while in the press.The mechanical cycle may vary, for example, from 20 to 40 minutes,depending upon the press and conditions used, while the heat cycle mayvary, depending on the heat transfer requirements which are related tothe number of sheets in the press and the use or nonuse of metal pressplates and other factors. The heat cycle is shorter where a catalystresin is used and longer where a noncatalyst resin is used; e.g., as lowas 3 minutes or lower to as long as 40 minutes or more.

In one embodiment, a high-pressure press was used with 50 sheets perpress opening with no press plates used, and with a silicone-type orother release paper used between the sheets. The overall mechanicalcycle was between 28 to 30 minutes. The press was closed at 70° C., thepressure reached was 35 k/cm², the heat cycle curing time was 12 to 14minutes for a noncatalyst MF resin, and the heat cycle temperature rangewas about 105 to 121° C. On reaching a temperature of about 116° C., thecooling of the press is started to bring the press to its startingtemperature. The resulting sheet material was a flexible andnonfrangible sheet of the type described.

FIG. 3 illustrates the process of securing the sheet material to aselected substrate, wherein the material 46 is passed through a pair ofopposing rollers 48 and 50, with low contact pressure applied to forcethe material 46 into contact with the rough surface of a compositewood-chip, resin-bonded particle board 52. The particle boardpreferentially is coated with a thin coating of 0.1 to 20 mils with anadhesive compatible with the adhesive in the sheet 46, andpreferentially the same or a similar adhesive; to wit, a PVA resinslightly wet or tacky adhesive surface. Also the use of the additionaladhesive coating is optional, and other adhesive materials, such asresorcinol resins or other natural or synthetic, contact adhesives used,like urea-formaldehyde resin adhesive alone or in combination withvinylacetate resins like PVA. The bonding may be carried out withoutsanding the back surface of the sheet and at room temperatures or lowtemperatures, as in the high-pressure process. Surface defects of theparticle board 52 are not telegraphed, or telegraphing is minimized,through the hard cured sheet material. Due to the low pressure andtemperature cycle used to bond adhesively the sheet material, the softparticle board 52 is not reduced in thickness nor weakened by theadhesive-bonding operation. The finished product 56 may be employed as akitchen or bathroom counter top or for furniture surfaces or other usesas desired.

FIG. 4 is a fragmentary cross-sectional illustration of the unitarylaminate product 56, wherein the particle board, composed of wood chipsbonded with resin 52, has the fully cured, wear- and stain-resistant,nonporous, resin-rich sheet material 46 adhesively bonded by adhesiveline 54 to the rough surface of the particle board 52, with thevinyl-resin adhesive 58 impregnated into the cured MF sheets asillustrated.

My fully cured thin sheet material with the adhesive-rich resin-richsurfaces and the finished laminate material prepared by the use of thissheet material provide superior and unexpected advantages over the priorart materials and processes.

What I claim is:
 1. A thin nonfrangible sheet material suitable for useas a surface covering, which material comprises a fibrous sheet materialhaving a basis weight of from about 15 to 180 gsm and impregnatedsubstantially through the depth with from about 20 to 70% by weight of acured thermosetting resin and characterized by one surface which is anonporous, adhesive-free, fully cured, resin-rich face surface, andanother surface which is characterized by an adhesive-rich back surface,the adhesive being compatible with the cured resin, the depth ofpenetration of the adhesive sufficient to permit the sheet material tobe flexible and capable of being handled without fracture thereof, theadhesive-rich back surface adapted to be bonded to a substrate, and theresin-rich surface adapted to be used as a face surface covering on asubstrate.
 2. The sheet material of claim 1 wherein the adhesive is avinyl-acetate resin.
 3. The sheet material of claim 1 wherein the resinis a melamine-formaldehyde resin.
 4. The sheet material of claim 1wherein the adhesive impregnates up to about 50% of the depth of thesheet material.
 5. The sheet material of claim 1 wherein the paper is analpha cellulosic paper.
 6. A laminate material wherein the adhesive-richsurface of the sheet material of claim 1 is bonded to at least onesurface of a sheet substrate material.
 7. The laminate material of claim6 wherein the substrate material is a composition board.
 8. The laminatematerial of claim 6 which includes an adhesive coating layer on thesurface of the substrate between the adhesive-rich surface and thesubstrate surface.
 9. The sheet material of claim 1 wherein the fibroussheet material includes a top transparent sheet material and anunderlying printed sheet material.
 10. The sheet material of claim 1wherein the sheet material is a cellulosic sheet material having athickness of from about 0.1 to 0.3 mm.
 11. An improved composition boardhaving a thin, cured, resin surface, which particle board comprises:(a)a composition board; and (b) a thin fibrous nonfrangible sheet materialbonded to at least one surface of the composition board, the sheetmaterial having a basis weight of from about 15 to 180 gsm andcharacterized by a top face surface which is heat- andtemperature-resistant and has a high abrasion, wear and scratchresistance, substantially the entire thickness of the sheet materialhaving a fully cured melamine-formaldehyde resin therethrough, the curedresin in an amount of from about 20 to 70% by weight of the sheetmaterial, with the back bonding surface of the sheet material being avinyl-acetate resin adhesive-rich surface, the vinyl-acetate resinadhesive impregnating the sheet material up to a depth of about 50%, theadhesive-rich back surface being unsanded and bonded to the one surfaceof the board by an adhesive layer.
 12. The composition board of claim 11wherein the adhesive layer comprises polyvinyl-acetate resin,urea-formaldehyde resin or a combination thereof.
 13. The compositionboard of claim 11 wherein the sheet material comprises an alphacellulosic sheet material and the sheet material has a thickness ofabout 0.1 to 0.3 mm.
 14. A thin nonfrangible sheet material suitable foruse as a surface covering on a composition board which sheet materialcomprises a thin fibrous alpha cellulosic sheet material having a basisweight of from about 15 to 180 gsm and impregnated substantially throughits depth with from about 20 to 70% by weight of a curedmelamine-formaldehyde resin and characterized by one surface which is anonporous, adhesive-free, cured, resin-rich face surface, and anothersurface which is characterized by an adhesive-rich, nonsanded backsurface, the adhesive composed of a polyvinyl-acetate resin, theadhesive impregnating the sheet material from the back surface up toabout 50% of the depth of the sheet material to provide a nonfrangiblesheet material with the resin-rich face surface adapted to be used as aprotective cover and the adhesive-rich back surface adapted to be bondedto the surface of a composition board.